1. Field of the Invention
This invention broadly relates to systems for spatial homogenization of light beams and more particularly to systems for homogenizing such beams, for instance laser beams, while at the same time an output is obtained with variable cross-section.
2. Description of Related Art
As it is well known, in many applications in which high intensity light beams are utilized, for instance laser beams designed for thermal and/or ablative treatments of surfaces, an accurate energy (or power) density on the working spot as well as a very high spatial uniformity are required, in order that any intensity fluctuations be maintained within a limit of 10%.
The radiation emitted by any light source has an intrinsic point to point disuniformity, particularly when a laser radiation is involved, due to the fact that the light beam as emitted by most laser sources has an intensity spatial profile that is not constant along a transversal direction, due to principles related to the physics of the sources of this type. The sole system adapted to enhance the spatial homogeneity of a light source without jeopardizing the emitted energy provides for optically manipulate the concerned light radiation by means of a lens and/or mirror system.
Many optical systems are commercially available adapted to intercept a light beam having known dimensions and to convert its current dimensions into other dimensions of pre-established values while at the same time they assure a high spatial uniformity, that is low point to point fluctuations of the light intensity.
Most of the existing optical systems adapted to operate in such a way as to make the intensity distribution of the light beam more uniform have the drawback that, when the energy (or power) emitted from the source is established, the final dimensions of the beam and consequently the exploitable energy (or power) density are strictly determined. As a consequence of this, the various applications of light beams with uniform intensity distribution require different dedicated optical systems depending on the desired final dimensions of the light beams.
In this sense, the present invention is innovative in view of its flexibility (due to the fact that, even if the parameters of its components are established, it enables the dimensions of the final beam to be varied in a large range of values, so that it can be exploited in application requiring spatial homogeneous light beams having variable dimensions and an arbitrarily selectable energy (or power) density) and also in view of its simplicity (as it requires the smallest number of lenses to achieve the above result).
Moreover, in the following it will be shown that the present system can optimize the correction of local lacks of uniformity of the energy density, thus achieving a better uniformity with respect to that obtained by using homogenizers which only correct the average lack of uniformity.
This invention is based upon the principle that the incident light beam can be divided into a number n of portions of rectangular cross-section by means of an optical system and such portions can be recombined in a suitable planexe2x80x94the so-called focal planexe2x80x94where the size of any individual beam portion coincides with the desired final dimensions, by means of two further optical components for each of two transverse directions. This process operates so that each point of the beam on the focal plane is the result of the combination of n different points of the input beam, thereby reducing the initial intensity fluctuations. The use of three optical components for each direction additionally enables the final dimensions of the light beam to be arbitrarily established, regardless of the starting dimensions, by simply modifying the relative distances of the above mentioned components.
The main applications of a spatial homogenizer for light radiation are those based upon the Interaction processes between radiation and matter in which a constant energy (or power) density is desired on the surface to be irradiated, among which, by way of exemplification, the processes for treatment of metals (inclusive of surface cleaning and/or hardening effects) and plastics (inclusive of writing, ablation and sculpturing), microlithographic processes on large areas, metal cladding and also all those processes designed to transfrom the structure of a material (such as the crystallization of amorphous silicon for subsequent utilization in photovoltaic applications and microelectronics) can be mentioned. The latter application has the most rigid requirements in respect of the spatial uniformity characteristics relating to the light energy (or power) density: in fact, intensity fluctuations lower than 5% are required.
The main advantageous characteristics of this invention reside on one hand in the possibility to continuously vary the dimensions of the light beam on the focal plane, so as to enable the desired energy (or power) density (that is inversely proportional to the dimensions of the light beam) to be precisely determined or the dimensions of the light beam to match the dimensions of the irradiated material, while on the other hand in the possibility of correcting the local intensity fluctuations of the input beam, to achieve a better uniformity with respect to that obtained by using existing homogenizers which only correct the lack of uniformity averaged over the whole beam.
Subject-matter of this invention is a combined optical system comprising six components each consisting of a number of lenses, adapted to transform the cross-section shape of a light beam into another shape having variable dimensions, as well as an enhanced spatial uniformity of the light energy (or power) density such as to enable it to be utilized in all processes requiring restricted intensity fluctuations on a pre-established area, or to realize a light beam having a constant intensity within an area of variable dimensions.
The above mentioned six optical components act on the two transverse directions of the light beam, three components for each axis, and they are so defined: the first component is defined by Horizontal Divider (Vertical Divider, for the other direction); the second component is defined as Horizontal Condenser (Vertical Condenser, for the other direction); and the third component is defined by Horizontal Zoom (Vertical Zoom, for the other direction).
The first component is comprised of a number of lenses (for instance cylindrical lenses), whose number, size and the focal length are to be established as a function of the desired results, while both the second and the third components are single lenses (for instance cylindrical lenses).
Let us consider the simple case when the Horizontal (Vertical) divider is composed by a number of equal lenses. Upon designating with f1 the focal length of the lenses of the first component, with f2 the focal length of the second component, with f3 the focal length of the third component, with d the distance between the second and the third components, with n the number of the lenses constituting the first component and with s the dimension of such lenses in the direction along which the focalization effect is exploited, the optical system operates so as to transform the starting dimension of the light beam in the considered direction, equal to nxc3x97s, into a dimension D, where D is defined by the following formula:   D  =            s              f        1              ⁢          "LeftBracketingBar"                                    f            2                    ⁢                      f            3                                                f            3                    +                      f            2                    -          d                    "RightBracketingBar"      
The dimension of the beam reaches a value D at a distance z greater than 0 from the third component according to the following formula:   z  =                    f        3            ⁢              (                              f            2                    -          d                )                            f        3            +              f        2            -      d      
where the values of f2, f3 and d are to be selected in such a manner as to determine a positive value of z.
Considered of this plane, for a length equal to D, the beam reaches a spatial uniformity of the energy (or power) density that is proportional to the number n of the lenses constituting the first component.
The above outlined system represents a broad arrangement comprising a number of particular cases among which it is possible to mention:
a case in which the lenses forming the Horizontal and/or the Vertical divider are not equal, that is, they have different sizes si (i=1, 2, . . . n). In the case, each lens of the Horizontal and/or Vertical divider must have a focal length f1 such that the ratio si/fi is constant for every i=1, 2, . . . n.
a case in which the lenses forming the Horizontal Divider and the Vertical Divider are parts of a single system of toroidal lenses or, when the focal length connected with the two directions are identical, spherical lenses (the latter optical component is generally defined as xe2x80x9cfly-eyexe2x80x9d);
a case in which the lenses forming the Horizontal Condenser and the Vertical Condenser are parts of a single system of toroidal or spherical lenses;
a case in which the lenses forming the Horizontal Zoom and the Vertical Zoom are parts of a single system of toroidal or spherical lenses;
a case in which the above mentioned cases occur contemporaneously with spherical lenses: in this circumstance, the optical system operates in such a way that the final dimensions of the light beam fulfill a pre-established proportionality relationship between the two directions.